The present invention generally relates to a technique and apparatus for optimizing the use of frequency spectrum allocated for the transmission of information and, more particularly, to a method and corresponding apparatus which allows two television signals to be transmitted using the same bandwidth normally used to transmit one television signal.
The general problem solved by the invention is to transmit two television programs simultaneously in the six megahertz channel space normally allotted for one program and allow reception of either program at each of a plurality of locations where the programs are received. This is particularly desirable in cable television systems in which a limited number of channels are available for transmitting television programs to subscribers. Several methods for achieving this improvement in spectrum utilization have been implemented in the prior art; however, each of the prior art methods have resulted in problems of performance and/or cost.
Typically, prior art systems used to process television signals have used a technique of sending one horizontal line at high bandwidth followed by limited bandwidth information which in combination with the first line can be used to form the second horizontal line. These techniques invariably generate visible artifacts in vertical lines that show up as alternate wide and narrow portions known as "zippering" or a stair-step pattern on slat lines known as "jaggies". The problem then becomes one of providing a bandwidth reduction system without easily noticed and objectionable artifacts.
In the typical prior art frequency division multiplex approach, it has been necessary to provide a single horizontal line of storage. A problem with this approach is the cost and complexity of producing a quality line of horizontal storage. For good quality imaging, a 910 pixel delay line with 100:1 or better dynamic range is typically required at the receiving locations. In the present system, however, the apparatus necessary to decode the transmitted signals is required to be a minimum cost item because it must compete in price with converters which have no processing. Prior art time division multiplexed approaches require two horizontal lines of storage because information is being displayed on the television screen at the same time new information is arriving in a time compressed form. When high resolution and long storage times are required, analog delay lines cannot be used because they tend to leak charge during storage causing a loss of resolution. Digital techniques can meet the performance requirements but digital delay lines presently cost five to ten times the cost of a charge coupled device (CCD) that is typically used as an analog delay line.
Because a television picture generally suffers some degradation when the video signal is processed through a system optimized to conserve spectrum, it is sometimes desirable to enhance the sharpness of the edges of the picture to preserve a crisp, sharp picture. In the prior art systems, the slant line artifacts (jaggies) are further enhanced when the sharpness of the picture is enhanced, making them more objectionable. The problem, more specifically then, is to provide a bandwidth reduction system which is amenable to edge enhancement without also enhancing visible artifacts.